(1) Field of the Invention
The present invention relates to a telescopic universal transmission joint, especially for motor vehicles.
(2) State of the Prior Art
FR-A-2,607,883 makes known a homokinetic transmission joint comprising a tripod equipped with three arms arranged substantially radially relative to its axis and each partially surrounded by two roller segments, radially outer faces of curved transverse profile of which are in rolling contact with longitudinal tracks formed on the inner face of a hollow element of generally cylindrical, or "barrel", form, surrounding the tripod.
During operation, the axial component of the movements of each arm of the tripod in the barrel, such as result from the telescopic movement of the joint or from an operating mode at an angle, is, in principle, converted into a rolling without sliding of the roller segments on their respective track. This rolling movement is such that, when one arm of the tripod is directed towards either one of its axial stroke limits in the barrel, the associated roller segments move aside towards the rear of the arm in relation to the direction of the axial movement in question. This makes it possible, in principle, to give the joint a reduced axial bulk for a given capacity for axial and angular movement.
Furthermore, FR-A-2,607,883 teaches, as an improvement, to extend the toroidal rolling surface of the roller segments at each of its circumferential ends by a tangential cylindrical surface having the same transverse profile as the toroidal surface. At the end of rotation, this cylindrical surface comes to bear and slide on the rolling track with which it mates. This increases the effective expansion and contraction stroke of the joint.
FR-A-2,622,653 teaches various arrangements for compelling each roller segment to execute its rolling movement without sliding when the roller segment travels along the associated longitudinal track.
These known joints have notable mechanical qualities. The rolling without sliding in the longitudinal tracks of the barrel eliminates friction, whilst the pivoting/oscillation combination between spherical bearing surfaces of the roller segments and the arms of the tripod represents a movement without a stopping point which is very favourable to hydrodynamic lubrication and to the self-polishing of the surfaces.
However, the rolling surfaces, that is to say the toroidal outer surface of each segment and the corresponding longitudinal track of the barrel, are in Hertzian contact, that is to say a contact theoretically along a line and in practice over a small area adjacent to this line, under a relatively high pressure attributable to the load to be transmitted. This makes it necessary for these surfaces to be very hard. Moreover, the load reversals capable of abruptly breaking and remaking this contact can be noisy. To remedy this, a high production accuracy or pairing during assembly can be adopted, but of course this is more costly.